1. Field of the Invention
The present invention relates generally to a spatial multiplexing detection apparatus and method in a Multiple Input Multiple Output (MIMO) system, and in particular, to a spatial multiplexing detection apparatus and method in a MIMO system, that can reduce the complexity of a receiver and prevent the performance degradation.
2. Description of the Related Art
To support high quality multimedia services as well as voice services, various technologies for next generation wireless communication systems have been developed for transmitting at higher speed more data with lower error probability.
Multiple Input Multiple Output (MIMO) systems use multiple antennas at a transmitter and a receiver. Compared with systems using a single antenna, MIMO systems can increase channel capacity in proportion to the number of antennas without additional frequency allocation or additional transmission power allocation.
MIMO technologies are classified into a spatial diversity scheme, a spatial multiplexing scheme, and a combination scheme of the spatial diversity and the spatial multiplexing.
The spatial diversity scheme is to simultaneously transmit a single transmission (TX) signal through a plurality of TX antennas. Thus, the spatial diversity scheme can obtain diversity gain corresponding to the multiplication of the number of TX antennas and the number of reception (RX) antennas, improving the transmission reliability. Alternatively, the spatial multiplexing scheme simultaneously transmits data streams thereby increasing data throughput.
As described above, the spatial multiplexing scheme transmits the independent data stream through the respective TX antennas. Interference occurs between the transmitted data streams because a plurality of data streams are simultaneously transmitted through the TX antennas. Therefore, the receiver detects signals, considering the influence of interference signal, by using a spatial multiplexing detection scheme, such as a maximum likelihood, a successive interference cancellation (SIC), and a Vertical-Bell Labs Layered Space Time (V-BLAST), or detects signals after removing the interference. The interference cancellation includes but, is not limited to a Zero Forcing (ZF), and a Minimum Mean Square Error (MMSE),
For purpose of explaining the spacing multiplexing detection algorithm, the MIMO system will be described herein with reference to FIG. 1.
FIG. 1 is a block diagram of a general MIMO system. Referring to FIG. 1, a transmitter of the MIMO system includes a Multiplexer 101, Inverse Fast Fourier Transform (IFFT) units 103, 105 and 107, and NT number of TX antennas 109, 111 and 113. A receiver of the MIMO system includes NR number of RX antennas 121, 123 and 125, Fast Fourier Transform (FFT) units 127, 129 and 131, and a Signal Detector 133.
In the transmitter, the multiplexer 101 multiplexes data streams to be transmitted to the receiver that include as many as the number of the TX antennas 109, 111 and 113. The IFFT units 103, 105 and 107 are provided at the TX antennas 109, 111 and 113, respectively. The IFFT units 103, 105 and 107 IFFT-process output signals of the multiplexer 101 to transmit the IFFT-processed signals through the TX antennas 109, 111 and 113.
In the receiver, signals are received through the RX antennas 121, 123 and 125 and are FFT-processed by the FFT units 127, 129 and 131 provided at the RX antennas 121, 123 and 125, respectively. The signal detector 133 processes the FFT-processed data streams.
The TX signal vector x=[x1, x2, . . . , xNT]T is transmitted through the TX antennas 109, 111 and 113 over a channel H, and the receiver receives a signal y=[y1, y2, . . . , yNR]T expressed as Equation (1) herein.
Equation (1) as expressed herein, shows the relationship of the TX signal and the RX signal in the MIMO system with NR number of RX antennas and NT number of TX antennas.
                              (                                                                      y                  1                                                                                                      y                  2                                                                                    ⋮                                                                                      y                                      N                    R                                                                                )                =                                            (                                                                                          h                                              1                        ,                        1                                                                                                                        h                                              1                        ,                        2                                                                                                  …                                                                              h                                              1                        ,                                                  N                          T                                                                                                                                                                                h                                              2                        ,                        1                                                                                                                        h                                              2                        ,                        2                                                                                                  …                                                                              h                                              2                        ,                                                  N                          T                                                                                                                                                          ⋮                                                        ⋮                                                        ⋱                                                        ⋮                                                                                                              h                                                                        N                          R                                                ,                        1                                                                                                                        h                                                                        N                          R                                                ,                        2                                                                                                  …                                                                              h                                                                        N                          R                                                ,                                                  N                          T                                                                                                                                )                        ⁢                          (                                                                                          x                      1                                                                                                                                  x                      2                                                                                                            ⋮                                                                                                              x                                              N                        T                                                                                                        )                                +                      (                                                                                n                    1                                                                                                                    n                    2                                                                                                ⋮                                                                                                  x                                          N                      R                                                                                            )                                              (        1        )            where y is an Rx signal, H is an NR×NT matrix, an element hij is a channel response between an ith RX antenna and a jth TX antenna, x is a TX signal transmitted through the respective TX antennas, and n is noise of the RX antennas.
Herein a conventional spatial multiplexing detection method in the MIMO system of FIG. 1 will be described.
First, applying Equation (2) below, a Maximum Likelihood (ML) receiver calculates Euclidean distance with respect to all symbol vectors existing in the channel arrangement of Equation (1). Then, the vector having the smallest Euclidean distance is selected.
Equation (2), as expressed herein is an equation for detecting the maximum likelihood.
                              x          ^                =                                                                              arg                  ⁢                                                                          ⁢                  min                                                                                    x                                              ⁢                                                                  y                -                Hx                                                    F            2                                              (        2        )            
Because all the symbol vectors are examined, an amount of calculation of the ML represents an amount of calculation of MNT. That is, the amount of calculation is exponentially proportional to the number of the TX antennas.
The SIC scheme is to remove a value detected at a previous stage from an RX signal. However, in the SIC scheme reliability of the previously detected value is lowered with the passing of each stage. Therefore, the SIC scheme needs to consider error propagation acting as a performance degradation factor. That is, due to the SIC process, the performance of a TX antenna signal having weak signal strength is not greatly improved.
The V-BLAST scheme is an improved algorithm of the SIC. The V-BLAST scheme performs the SIC process in the order of TX antenna indexes having large signal strengths. The V-BLAST schemes performance is more improved than the existing SIC scheme.
A Modified ML (MML) performs the above ML with respect to the symbol vectors transmitted from all TX antennas, except a signal transmitted from one TX antenna. The signal transmitted from single TX antenna is sliced and calculated using Equation (3) as expressed herein.
                              x          i                =                  Q          ⁡                      (                                                            h                  i                  H                                                                                                                h                      i                                                                            2                                            ⁢                              (                                  y                  -                                                            ∑                                              j                        ⁡                                                  (                                                      ≠                            i                                                    )                                                                                      ⁢                                                                  h                        j                                            ⁢                                              x                        j                                                                                            )                                      )                                              (        3        )            where i is one TX antenna, j is the other TX antennas,
  y  -            ∑              i        ∈                              {                          1              ,              2              ,                                                          ⁢              …              ⁢                                                          ,                              N                T                                      }                    /                      {            j            }                                ⁢                  h        i            ⁢              x                  i          ,                      N            T                              is a removal of TX antenna signals calculated through the ML.
The MML has the same performance as the ML, and the calculation complexity is reduced to MNT−1.
The performance of the spatial multiplexing receiver is inversely proportional to the calculation complexity of the receiver. That is, as the calculation complexity of the receiver is lower, the performance of the receiver is improved. However, because the ML or MML has high calculation complexity, there is a need for an algorithm for reducing the amount of calculation.